This invention relates to a method of depositing thin films, particularly thin films such as metallic films, in the fabrication of integrated circuits.
Present trends in the formation of vacuum deposited thin metallic film commonly use chemical etching in the presence of etch-resistant masking layers to provide the selected pattern. This is the traditional photoengraving or photolithographic etching technique. However, with the continued miniaturization of semiconductor integrated circuits to achieve greater component density and smaller units in large-scale integrated circuitry, the art is rapidly approaching a point where such photolithographic etching of deposited film may be impractical for providing the minute resolution required for the fine line work of metallization in circuitry where film widths of 2 microns or less are desired.
An alternative method for forming such metallization denoted by the term "expendable mask method," "lift-off method" or "stencil method." U.S. Pat. No. 3,873,361, issued Mar. 25, 1975, to Franco et al., entitled "A Method of Depositing Thin Film Utilizing a Lift-Off Mask," and assigned to the present assignee discloses a lift-off method for depositing thin films which avoids the "edge-tearing" problem and is suitable for use where the lateral widths of the spacing between adjacent deposited metallic lines is of the order of 0.05-0.25 mils. The method disclosed in this patent includes the use of an organic polymeric material deposited on the integrated circuit substrate and an overlying layer of an inorganic material, preferably metal, having openings in the selected pattern. Openings are formed in the polymeric material by reactive sputter etching utilizing the metallic mask as a barrier. The openings in the polymeric layer are aligned with and laterally wider than the corresponding openings in the metallic masking layer as a consequence of the reactive sputter etching step. Thus, the edges of the openings in the metallic masking layer overhang the edges of the openings in the underlying polymeric layer. The thin film to be deposited is then applied over the structure and on the surface of the substrate exposed by the openings in the polymeric material. When the polymeric material is removed by application of solvent, the metallic masking layer and the thin film above the masking layer "lift-off" to leave the thin film deposits in the selected pattern on the substrate without "edge tearing" of the desired deposited thin film as the unwanted portions of the thin film are lifted off.
With this process, employing a metallic reactive sputter etching mask, alignment of the etching masks which are used for forming apertures in the masking layers was made difficult as the metal layer was, of course, opaque. The alignment problem could be alleviated by providing two alignment areas at opposite ends of the integrated circuit substrate which are left unmetallized during the metal mask deposition. Unfortunately, the alignment areas are not then available to be used for the production of active surface components, thereby reducing the amount of circuitry which can be provided upon a predetermined wafer area. Also, the use of the evaporated metal reactive sputter etching masking layer requires the use of relatively expensive and time-consuming evaporation steps and the subsequent chemical etching step to pattern the evaporated layer.
To alleviate the problems attendant with the use of such a metallic etching mask, a process was developed in which the metal mask layer was replaced with a transparent layer of polydimethylsiloxane resin which permitted easy optical alignment and eliminated the need for yield-reducing dedicated alignment areas on the surface of the substrate. This method was described in U.S. Pat. No. 4,004,044, issued Jan. 18, 1978, to Franco et al., and assigned to the present assignee. Basically, in this method, the polydimethylsiloxane resin layer which replaced the metal layer was spun-on over the first polymeric masking layer. Otherwise, the method was the same as that described in earlier U.S. Pat. No. 3,873,361, discussed above.
The later-developed technique did in fact make it possible to optically align the exposure masks and eliminated the need for reserved alignment areas, thereby increasing the usable substrate area. Also, the use of such a material proved to be reliable and relatively simple to use in a manufacturing environment. However, it has recently become desirable to provide even finer resolution than is possible with the use of polydimethylsiloxane resin materials.
To produce even finer resolution, such as, for example, metal lands of less than 2 micron widths with 1 micron spacings, the "process bias" requires reduction over that which may be achieved using the polydimethylsiloxane resin material. "Process bias" of the lift-off process described in the patents referenced above is defined as the difference between the dimensions of the developed image (that is, the dimensions of the aperture produced in the upper or second masking layer directly from the photolithographic mask) and the dimensions of the metal film pattern produced at the final step after removal of the masking layers. For minimum process bias, it is desirable that the etch rate of the center layer be much lower than the etch rates for the first and second masking layers of photoresist material for the etchants used in the reactive ion etching steps of the first and second layers, while the etch rate for the center layer should be much higher than the etch rates for the first and second layers for the etchant material used in the reactive ion etching of the center layer. Also, for minimum process bias, it is desirable that the center layer be conformal, that is, the thickness of the center layer should remain the same no matter what the topography of the underlying layer.
Polydimethylsiloxane resin unfortunately has a relatively slow etch rate even with its preferred CF.sub.4 plasma etchant. In fact, it is about equal for this etchant material to the etch rate of the imaging resist material. Thus, when the prior art polydimethylsiloxane resin material was etched, the aperture formed in the upper second masking layer was also affected, thereby changing the dimensions of the thin metal film substantially from that which was intended from the dimensions of the original photolithographic mask. Even if the dimensions of the photolithographic mask were decreased to compensate for the relatively slower etch rate of polydimethylsiloxane resin, it was not possible to produce a very precisely dimensioned layer of metal film as its dimensions were then a function of processing variables other than the photolithographic mask dimensions, such as etching time, thickness of the layer of polydimethylsiloxane resin, substrate topography and temperature. Moreover, because polydimethylsiloxane resin must be deposited using a spin-on technique, it was non-conformal and thus subject to being coated on with a thickness which was greatly dependent upon the substrate topography. For example, over a via hole, the thickness of the polydimethylsiloxane layer could be more than twice that in planar regions.